Method of forming wound magnetic cores



Oct. 20, U s DUNN METHOD OF FORMING WOUND MAGNETIC CORE-S 4 Sheets-Sheet 1 Filed March 24, 1947 INVENTOR. 00055.: J. DUNN ATTORNEYS U. S.DUNN METHOD OF FORMING WOUND MAGNETIC CORES Oct. 20, 1953 4 Sheets-Sheet2 Filed March 24, 1947 j INVENTOR.

U1. r5555 5. 0u/v/v W Wm A TTORNE Y5 Oct. 20, 5 DUNN METHOD OF FORMINGWOUND MAGNETIC COREIS Filed March 24, 1947 4 e s- 3 INVENTOR.

- U1. V6566 5. DUN/v Oct. 20, U. S. DUNN METHOD OF FORMING WOUNDMAGNETIC CORES 4 Sheets-Sheet 4 Filed March 24, 1947 a 1 a U C; 1/ IINVENTOR. ULYSSEJ 5. Dumv M 'fl-Wr M ArroR/vns A I/I a UNITED STATESPATENT OFFICE 2-,'s'55,71-i here-ob, oe .rloamiso wouso MAGNETIC cormsUlysses s. mmh; Clii'cai'o', in. Application March 24, 1946, Serial llo. z'o'ieimez (01. 29-15558) This invention relates to the winding ofmag: netic' strip material into cores for transformers and otherelectrical apparatus and is particularly' concerned with novel methodsof winding the core and assembly or the wound core with e wound coil orcoils in the manufacture of eieo-' trio'al equipment. v

The invention 1h as srererreo embodiment comprises the winding of stripsteel or use metal of high magnetic permeability under tension into asuitable core having the form of a closed figure, cutting' the core openat ne place for as;-- ser'nbling it with re -wound coil which also is inthe form or a noted fl'urear'id 1s uncut, and restoring the core to itsoriginal wound shape interlinked with the eon.

It has heretofore been piopo sed to provide as-' sem'biie's' of woundcores or strip ih'e'tei and associated coils in transformers and likeelectrical instruments in merry dlfi're'ii't simmers. A widely usedcommercial method or assembly has been to Wind the core W'itli 8'.bonding material between the winding layers to prevent the core fromfalling apart when out in two places, and cut the wound core into twoseparable pieces which are then reassembled to enclose the cut woundcoil, the core thus having two b'utt Joints in the finished roduct. I

A diniculty with this method of assembly is that it is necessary toprovide special bonding material that is not adverselyafiec'ted by thehigh annealing teniperatur'e's'jt'o' which the core is later subjected.In rhy inventi n I are able to dispense with the use of such bonding"r'n'at'eifiall Another objection to this" method of assembly has beenthat the provision of two butt joints in the core provi'des' doubleopportunity for undesired air gaps in the magnetic circ'uit, therebyreducing the efliclehy of the magnetic circuit.

A further difficulty is that quote inherent limitations in eccureoy'mcutting the oore into two parts it is always necessary to keep thecorresponding severed coreparts matched together" for subsequentreassembly because parts of two different cores are usually notinterchangeable.

Besides, there are difficulties of assembly in aligning the adjacentends of matched core parts, the core bond may be fractured duringhandling or assembly in which case thecore' parts will not snugly fittogether again so as toallow the butt joints to close adequately, andthere is always danger of fracture of the core at the butt Joints duringincorporation of the coreandcoil assembly into an instrument orduringuse of the instrument.

sieseht i vehafoh relates generally to this type of assembly but,instead of having to out the core so as to produce two butt joints, I amenabled to assemble eel'o's'eu wound core with a cllis''dwdund 6011while making only one butt joint in the sore, this joint being solocated on the core as to enable the wound core to be sprung' apart forinsertion of the coil on one leg th ereofwithout damagin the coil andwithooth'eh eheht y defiir'fiiiii fhe core, and be refor'r'ned with soefficiently closed butt joint.

With the abovein mind, it is a major object of invention tb'provide anovel method of winding magnetic strip material into a core in such amanner that only a single out need be made 11; the o'rfe forinterimrihig assembly with a c osed wound coil; and t e novel core sowound.

It is e furltheif obieot ofniyinVention to provide a novel methodofwin'dir'ig' magnetic strip material un er ension iiit'da closed corewhere mthe winding t'eheioh' is' iiiedueuy reduced dur ing thewindingoperation, and the novel core thus formed which may out through toprovide a single bu'tt J'Q hflfha't closes fliciently' through the ehr'e petrr or the out.

It is a further object or invention to provide e novel method of windingmagnetic strip lrrateria l into' a closed one for transformers or likeelectrical eq'ulpmen't' wherein the core is wound on a rotating 'formunder gradually and 'co nstantl cli'anjgiin'g strip tension whichlessens as the" s 2e of the wound core increases. Pursuantto this objectprovision may be made for automatically decreasing the winding tension,as by decreasing it in sillta'bl' increments as each turn is wound onthe core.

A, furtherobject of my invention is to provide a novel wound ooreoi'magnetic strip material adapted for association with a closed pre-woundcoil whe'r el'n hee'ojre ha only a single butt Joint located at an'o'othhuiq position for assembly with the coil without injuriouslydeforming th core that is deforming or bending the core pqd. he ielpoint A further object 01 myv invention is to provide s ovel assembly ofa generally oblong wound strip steel core with a closed form wound coilwherein the core has onlyia single butt joint located at a" cornerthereof in predetermined relation to the legon whieh the coil is mountedIt is a further object of my invention to provide a novel method ofcutting a closed wound core. in such a manner 8 to prevent separation ofthe turns d rin cutt so as to prevent formation of burrs betweenturnsand to provide a" comparatively smooth cut to give a joint of highmagnetic efficiency. and apparatus holding it during cutting.

A further object of my invention is to provide a novel method of windinga core of strip magnetic material which is to be cut at one portion toprovide a single butt joint when assembled with a form wound coil,wherein the core is initially wound with an oversized dimension at saidportion so as to compensate for the change in dimension in the closedcore after material cut out to make the butt joint is removed from thecore.

A further object of my invention is to provide a novel method andapparatus for preheating strip metal as it is wound upon a form to makea closed core to eliminate subsequent annealing of the wound core.

Further objects of the invention will presently appear as thedescription proceeds in connection with the appended claims and theannexed drawlngs wherein:

Figure l is a front elevation of a simple transformer core and coilassembly having a core wound according to a preferred embodiment of theinvention;

Figure 2 is a side elevation of the assembly of Figure 1;

Figure 3 is a diagrammatic representation of the preferred method ofwinding the core of Figures l and 2;

Figure 4 is an elevation partly in section illustrating an annealingfurnace for a plurality of wound cores prior to cutting;

Figure 5 is a plan view illustrating a clamping fixture for holding andcutting the wound core;

Figure 6 is a side elevation of the wound core of Figure 1 after cuttingat the corner;

Figure '7 is a diagrammatic elevation illustrating separation of theouter turns of a core wound under constant tension;

Figure 8 is a side elevation illustrating how the core is sprung toreceive the coil during assembly;

Figure 9 is a side elevation illustrating the use of a wedge-shapedwinding form which is enough wider at the end where the cut is made tocompensate for the width of the cut whereby the finished core aftercutting is the same width at both ends;

Figure 10 illustrates in elevation a choke coil reactor containing acore made according to the invention;

Figure 11 is a side elevation, partly in section, of a "shell" typetransformer having a core embodying two core parts each wound accordingto the invention; and

Figure 12 is a side elevation in section diagrammatically illustratinga-further embodiment of the invention wherein the strip is wound hot, aprocess by which the subsequent annealing operation may be eliminated.

Figures 1 and 2 illustrate a typical core and coil assembly madeaccording to the invention. The assembly comprises a closed oblong coreI I wound of magnetic strip material such as high silicon electricalsteel, a closed form wound electrical coil l2 which is mounted on thelong leg of core I i surrounding and enclosing it, and a core clampcomprising a thin flexible metal band [3 having its opposite endstightly drawn together by suitable fastening means such as indicated atH for drawing the core ends together to tightly close the butt joint [5at the upper left corner of the core in Figure 1. Above and below thecoil [2 are preferably provided protective strips of insu- 4 lation orlike material l6 and I! which also serve as spacers and to hold the coilagainst undesired movement along the side leg of the core. Insulationstrip l6 also serves to hold the joint (5 in alignment as band 13 istightened during assembly.

Core II is wound from continuous strip electrical steel that is of highsilicon content, preferably in the present commercial range of about 3to 6 percent silicon, which is of a requisite quality and possessesspecial magnetic characteristics commonly known to such core steel inthe art. Strip steel of this kind having a non-conducting surfacecoating to reduce interlamination eddy currents and processed to have adesired grain orientation so that it has high magnetic permeability inone direction lengthwise of the strip may be obtained as stock materialfrom several of the large steel companies, the characteristics varyingaccording to the treatment given the sheet by the differentmanufacturers. It is believed sufiicient for purpose of this inventionto state that preferably core H is made of winding continuous strip I ofelectrical steel having desired magnetic properties.

Referring now to Figure 3, the core I l is wound on a form block 18which is removably secured to a rotating head [9 that revolves about acenter 2| being driven by a suitable lathe motor or the like (notshown). Suitable dowels 20 prevent relative rotation of the block andhead l9 and help support the block on the head. The peripheral shape ofblock l8 of course determines the shape of the wound core. In theembodiment selected for illustration, as for winding the core of Figure1, block [8 is rectangular, having slightly rounded corners tofacilitate the winding of the steel which winds smoother and tighteraround rounded corners. In practice the corner where the core is to becut may be fairly sharp and the other corners may have a much largerradius especially the corners opposite the coil, and the block isprovided with suitable slot or other means on its periphery foranchoring the 7 leading end of the strip material 22 to be woundthereon.

Material 22 is uncoiled from a large roll of strip material 23 suitablymounted on a conventional expansible reel 24, and strip 22 passes inturn through an adjustable tension device 25 provided with a manualcontrol 28 later to be explained, and a pair of hardened pressurerollers 21 and 28, the upper of which is mounted in a slidable supportwhereby it may be moved toward or from the strip 22. Preferably frictiondevice 25 comprises a stationary block and a block 30' movable towardand away from strip 22 by manipulation of control 26, adjacent faces ofthe blocks 30 and 30 being provided with friction coverings engagingopposite sides of the strip. The purpose of rollers 21 and 28 is to rollout any burr or imperfection on the edges of the strip formed byslitting of the relatively narrow strip 22 from wider rolled sheetmaterial.

Strip 22 then passes through a set of straightening rolls 29 which alsoserve as guide rolls for preparing the strip for entry to the windingapparatus in alignment with the rotating block [8.

Instead of using a form block [8 as in Figure 3, a two jaw chuck may beused and the coil wound around the jaws. The exterior of the jaws arecontoured so that when expanded to the required size they will make aform providing a window in the coil and havingthe requiredcharacteristlcs described in connection with the block [8. around thejaws to form the core until the desired number of turns are obtained.The same principle of holding the free end of the coil, after the stripis severed is used. The chuck jaws are then retracted and the woundco're removed. Due to the tension in the windings the core will bulgeoutwardly so that the window is shortened. Before annealing, which willbe described, a block or pair of tapered blocks are inserted into thewindow to return the core to the required size and also to provide ameans for pressing the core for its annealing operation. By this methoda block i8 is not needed for each core as it is wound, and only enoughof the described tapered blocks are required as are needed for thenumber of cores being annealed at one time.

A suitable adjustable friction brake (not shown) is provided on the reel24 to prevent uncoiling of the strip from the reel except when drawntherefrom by the pull of winding block I8. This may be of the usual typeof unreeling apparatus. Strip tension control device 28 frictionallygrips strip 22 in opposition to the pull exerted by rotating windingblock l8, thereby controlling the winding tension. This winding tensionmay be varied at will by the control 26, and reliable and predictablecontrol of this winding tension during the winding operation is a veryimportant feature of the invention, as will presently appear. If desiredadditional strip tension control could be obtained by adjusting thebrake at reel 24. In practice the tension at reel '24 is comparativelysmall as compared to the maximum tension at tension device 25 in orderto permit a high ratio of total tension variation.

In practicing the invention, I prefer to decrease the winding tension ofstrip 22 gradually as the core turns increase in number on block i8.This is preferably done in the following manner. When winding of thecore is started with the leading and of strip 22 anchored on block (0, Iadjust the frictional gripping force at device 26 by manipulation ofhandle 26 to a practical limit which is usually determined by thetendency of strip 22 to pull loose from the point of attachment to blockI 8. In other words, I initially provide substantially maximum tensionon the strip permissible with winding of the strip. Then rotation ofblock 18 is started and as the core turns are wound onto block i 8, thetension on the strip exerted by device 25 is gradually reduced. This maybe done either manually, as by manipulation of handle 26, or, ifdesired, a suitable automatic means may be provided operating in timedrelation with the rotation of block ill for releasing tension of thestrip as the core turns accumulate. Where an automatic tension releasingdevice is provided at 25, it can be timed to release a predeterminedincrement of tension for each wound turn of the core. The initial andfinal tension will vary and are related to the core size as determinedby trial during the winding, annealing, and cutting operations of thefirst cores. In general however, -I have found that this gradual releasein tension of the strip during winding is of such degree that tensionexerted on the strip when the core has been finally wound on block i8 isroughly about one-fourth as great as the initial tension when thewinding was started. The advantage of winding the strip in this mannerwith constantly decreasing tension will be de- The chuck is rotated andthe strip wound scribed later on when its assembly with the coil isdescribed.

After a suitable length of strip material 22 has been wound on block I8, strip 22 is severed and the block is pulled axially oil the lathehead l9, the free end of strip 22 of the wound core H being boundtightly against the core body by a twisted wire loop H (see Figure 4)that extends throlugh a suitable notch 32 in one end of the bloc Thewound core is then ready for annealing. In annealing the core, it isplaced in a suitable clamping frame which tends to hold it in the shapewhich it is to assume in the finished product, and the wound core ll maybe annealed either separately, or in a group as in Figure 4. In Figure4, a plurality of the wound cores, still on their blocks 18 as removedfrom the lathe head of Figure 3, and with their outer ends tied by wireloops 3!, are held together between two compression jacks 33 and 34within a suitable furnace 35 wherein annealingheat is applied to thewound cores in a suitable manner. As illustrated in Figure 4, Jacks 33and 34 are mounted for adjustment towards and away from each otherwhereby the series of wound cores being annealed may be rigidly clamped,the adjacent side of each forming a support for the other, whereby thecore shape is maintained during the annealing process and given apermanent set by reason of the annealing process.

The purpose of the annealing under pressure is chiefly to remove strainsset up in the strip material by the slitting, winding and othermechanical operations which have been performed on the material so thatit is given a permanent set to the required shape and also improve themagnetic properties of the finished core. Preferably the annealingfurnace 35 has a temperature range of about 1500 to 1700 degrees F., andfor most steel wound into core form, I prefer to anneal the cores byheating them within the furnace to about 1700 F. for one hour, and thencooling them slowly to below 500 F. while still maintaining them clampedin the jacks 33 and 34 before removing them from the furnace. The exactannealing temperature will vary somewhat with the different grades ofsteel as may be recommended by experience or the steel manufacturers.The annealing and cooling time varies with the size of the core andwould be greater for a larger core, and are usually determined best bytrial.

Annealing procedures have been largely standardized for various gradesof electrical steel and it is necessary only to briefly discussconditions of this practical step. Where the core is wound of thestandard grades of hot rolled silicon steel. the furnace atmosphere isnot critical. Neutral to slightly oxidizing atmospheres give verysatisfactory results. A carbonizing atmosphere is to be avoided as anincrease in carbon content increases the magnetic losses. Certain gradesof high silicon steels, especially those cold rolled to produce highmagnetic permeability parallel to the length 0'1 the strip have inherentcharacteristics which may be adversely affected by excessive annealingtemperatures or an incompatible furnace atmosphere. For example, ahighly oxidizing atmosphere, especially when combined with temperatureabove 1700' F. increases the magnetic losses and produces a heavy oxideon the edges of the core layers so as to cause the'core to be bondedbetween layers by the oxide which makes the core so stiif that it is'dimcult to spring apart for assembly of the coil in the process laterto be described and equally difficult to close again with an eilicientbutt Joint. If the atmosphere is excessively reducing, such may causethe edges of the wound sheet steel to adhere metal to metal betweenlayers so as to cause the same difficulties in opening and closing thecore, with the further difficulty that magnetic losses due to eddycurrents would be greatly increased. Hence it is most practical for allgrades of steel to main taln a completely neutral atmosphere around thecores while annealing. A mixture of 90 percent nitrogen and 10 percenthydrogen flowing through the heating chamber gives a practically neutralatmosphere even with incidental leakage of free air. A vacuum in theannealing chamber gives a strictly neutral condition. Hence it is theoptimum condition of annealing at 1500 F. to l700 F. in a neutralatmosphere that is recommended for practice of the invention.

The annealed core removed from the furnace now has its permanent shapeand the wire loops 3| may be removed from the ends thereof without thestrip 22 uncoiling. The problem now is to assemble the closed wound corewith a prewound closed annular coil of wire which, in the preferredembodiment of the invention, is intended to surround long leg 36 of coreII. In order to do this, I cut the wound core entirely through at thecorner adjacent one end of leg 38 for the purpose of springing leg 36out in order to slip the annular coil 12 thereover during assembly.

Figure 5 illustrates a Jig for holding the wound core while cuttingthrough the core corner for this purpose. This jig comprises a base 31which is a steel plate of required size. Core H is mounted on base 31for sawing, being rigidly held in place on block 31 by bolts 38 thatpass through the dowel pin holes of winding block l8 and are threaded attheir lower ends in plate 31. While I prefer to employ the originalblock l8 for holding the core rigidly in sawing position, any equivalentmounting arrangement may be provided on plate 31, as for example a blockintegral with or permanently secured to base 3'! and shaped like block[8 to be received by the wound core. At one corner, base 31 isdiagonally slotted at 31 to allow a saw to enter and be correctlylocated and guided with respect to the cut to be made in the corner ofcore ii. At the same corner of the base are a pair of side by side Jawscomprising blocks 39 and 4| slidable parallel to each other and slot 31'for engaging the outer periphery of core H at its diagonal corner and onopposite sides of and very close to the desired saw cut location. Thedimension of blocks 39 and 4| normal to the plane of the paper ispreferably at least equal to the width of strip 22. Blocks 38 and 4! arearcuate at their inner ends to fit snugly with the wound core periphery.Adjustment screws 42 and 43 are threadedly mounted in stationarysupports on base 31 and rotatably connected to blocks 39 and 4|, wherebywhen the screws are turned in the same direction, blocks 38 and 4! willbe moved inwardly against the periphery of wound core I I as illustratedin Figure 5. The purpose of blocks 38 and 41 is to clamp the corewindings very tightly on opposie sides of the line where it is to be cutand as near as possible thereto. This is important in order to reducethe formation of burrs and irregular edges at the cut portion.

Besides being the optimum place for cutting the 8 require a minimumdistortion of leg 36 to receive the coil, the corner has been found tobe the best place to cut an oblong core because it is there that thematerial can be clamped under maximum core for threading the coil ontoleg 38 so as to tension for cutting, and this the place that the Jointcloses with the least movement of the two intersecting sides asdetermined by the geometric proportions of the core. An analyticalconsideration of the process of winding with decreasing tension followedby clamping and annealing under pressure shows that the corner has aninherent advantage as the place to make a cut that will close again togive a satisfactory closed butt Joint. The process of winding withdecreasing tension provides a minute amount of space or slack betweenturns which increases toward the outside layer. When the core is clampedand annealed under pressure as shown in Figure 4 this graduated space orslack between adjacent turns is forced entirely into the end sections.This is demonstrated by the fact that the dimensions 13 of Figure 6 arealways greater than dimension A in the annealed wound core. Thecircumferential clamping action of band i3 displaces this excessmaterial in the end portions, especially in the end that is cut, so asto tightly close the joint at IS. The method of assembling the coil ontothe core, the method of clamping and sawing, the natural geometricproportion of the core, and the spacing of the turns at the end due togradual winding tension followed by clamping and annealing all indicatethat these steps result in a wound core of the most practical form andhigh efficiency when the cut is located at the corner as shown. Hence,in the preferred embodiment of the invention, I make a saw cutexaggeratedly indicated at 44 in Figure 5 through the entire wound coreat the selected corner. In oblong core II, the cut 44 is preferably at45 to the straight sides of the wound core. As this saw cut 44 isindicated enlarged for purposes of clarity in Figure 5, it will be notedthat it is disposed between the pressure blocks 39 and 4!, whereby thecut core winding ends on opposite sides of the saw cut will be heldunder such considerable pressure that there will be little or norelative movement between the core winding layers during the cutting,and it is possible to make a sharp smooth out which will provide a buttjoint having an optimum chance of closing tightly when the core isclamped together in the complete assembly.

It is immaterial what type of instrument is used to make the cut for thecore at 44. Chiefly I have found it advisable to use a metal band sawwhich gives a joint that is good enough for most practical purposes,although it may be an abradant saw, friction saw, or any other saw orlike cutter. The chief thing to observe is that the out should remove aslittle material as possible because it will be appreciated that suchwill change the dimension of the core and thereby reduce the chances ofgetting an efilcient butt Joint when the core is clamped together in theassembly. A manner of compensating for excess material removed by such acut will be described in connection with Figure 9.

Figure 6 illustrates the core after it has been cut and removed from thefixture of Figure 5. As a result of the core being wound under graduallyreducing tension, it will be noted that the butt Joint I8 is capable ofclosing substantially completely and is magnetically very efllcient. Hadthe core been wound on form I8 in the proc- 688 0! ure 3 with constanttension on the strip material at all times as has heretofore 9 beenaccepted practice, the inner lamlnations of such a core H would be muchmore loosely wound than the outer laminations due to accumulatingtension in the strip as the core turns accumulated on block i8 whereby,after the corner cut was made in the fixture of Figure and the coreremoved therefrom, the parts would tend to assume the positionillustrated in Figure 7 wherein the more highly tensioned outerlaminations of the core would tend to relax and the fact that the outerturns have an ever increasing corner radius increases the tendency forthe outer turns to relax or straighten at the corners and thereforedecrease in length more than the inner turns which are held in plaoe'bythe more nearly square corners as well as the combined pressures of theouter turns whereby the resulting non-uniform wedge shaped gap at 44could never be completely closed as a butt joint. Hence the gap 44'would remain partly open, thereby providing an air gap in the magneticcircuit and a consequent loss of efllciency in the transformer due to agreatly increased exciting current.

By providing constantly decreasing tension during the winding operationas above explained, I insure that after the cut in the core has beenmade and the laminations allowed to assume their final positions, thereis no such relative shrinking oi the different laminations as in Figure7 but instead there is excess material in the end portions representedby minute space between turns which space increases towards the outerlayers in such a manner that this excess material is displaced by itsinherent pressure and by the pressure or the clamping device I3 so thatthe ends of the respective layers are readily closed to form a smoothbutt Joint as in Figure 6. This is an important feature of the inventionas has been confirmed by alternating current magnetization tests onvarious sizes and grades of cores throughout the useful range ofmagnetic densities. These tests show that cores made in the mannerdescribed and cut with an ordinary metal band saw have a total core lossin watts per pound not greater than and often less than the steelmanuiacwrers' currently published guarantees based on the idealconditions of the Epstein test. The exciting current in volt-amperes perpound is less than the manufacturers currently published data based onEpstein tests especially above a density of 10,000 gausses. This is ameasure of the practicability oi this invention which is technically andcommercially competitive with wound cores made with two or more buttjoints or with uncut cores which are annealed and then uncolled andrewound to thread them through a form wound coil turn by turn.

The wound, annealed and cut core i l of Figure 6 is now assembled withthe transformer or other coil i2. This assembly is accomplishedaccording to the method illustrated best in Fig- 11-. 8 wherein the leg36 is simply sprung out- Wind to provide a gap 45 at the corner largeenough to enable coil I! to be siidubly inserted onto the end oi leg 88and pushed: therealong until. when leg 38 is released the parts springback to approximately the positional Figure 1. Then to hold the core:closed a suitable steel strap or like clamp I3 is mounted about the com.and fastening, means. i4 manipulated to draw it as tight as possible tosecure maximum closure.- and efficiency: of the butt Joint it. Theinsulation strips, [8 and H may be inserted at any time during thisoperation.

- vibrations.

10 While the above described saw cut is described 'only as being made atthe optimum position of 45 degrees to the straight sides of the core, itwill be appreciated that as a practical matter this angle may be variedsomewhat within certain practical limits for obtaining equivalentresults without departing from the spirit of the invention. For example,referring to Figure 6, I may advantageously cut the core anywhere withinthe 45 degree angle between the optimum 45 degree illustrated Joint l5and the dotted line 45 parallel to leg 36 and intersecting the innercorner. -yond line 48 across the short leg of the core, as at the dottedline 41. because such would make the end of leg 88 larger than the crosssection of the leg and too large to accommodate the It is preierable notto cut the core becoil. It is also preferable not to cut the core acrossthe long leg 38 as at dotted line 48 because suoh would require leg 36to be bent too far outwardly to clear the short leg and receive thecoil. Ii the cut exceeds an angle of 45 to the inner surface 01' leg 88it is necessary (or the end portion of the core to bend downward inorder to close the butt Joint, whereas it the cut is made preferably atline I l but not beyond line 48 the entire deflection necessary to closethe Joint comes in the longer legs of the core.

I have investigated the efiect oi cutting core H at the various linesll, 48, 41 and 48 as well as across line 49 similar to line 48 but onthe opposite side of the corner parallel to the short 1 leg of the core.

that some oi them, particularly the inner turns, were loose and subjectto vibration. Where the major clamping force exerted by strap is wasnormal to the butt joint line in such unequalized pressure Joints, thejoints were tighter than in other methods of clamping but there was muchtendency toward vibration.

Hence as a practical matter and a result of observation I prefer to makethe saw out exactly diagonally of the corner or the coil and ii that isnot possible somewhere between the diagonal and a lin 48 parallel to thelong leg of the core and substantially coincident with the innerperiphery of the lon leg It will be understood that these limits are byno means restrictive or the invention but they are described herein toinform as to the best practical aspects of the invention which applicanthas found from experienc and tests.

As motioned above, the amount oi material which is removed at the sawcut 44 is important as a practical matter in that it may change thedimensions of the core, and this change of dimension becomes evidentwhere a wide blade saw is used that removes as much as up to aonemtoenth to one-eighth of an inch wide section across the core. Insuch cases I have iound it advisable to employ a suitably shaped windingblock N, which is trapezoidal in Figure 9, being wider at the short endadlacent which the out 44 is to'bemade. As winding of the strip 21prograsseson such a block the core is deformed so asiidbe-wider atcneend than the other, but this deformity is corrected after the saw cuthas been made and the butt joint formed, because now the leg 36 and theadjacent short leg of the core approach each other to such extent thatthe opposite legs of the core become substantially parallel in the finalassembly. The difference in length between the short sides of the block51 is therefore a function of the width of cut 44.

Figure illustrates an electrical instrument made according to theinvention wherein a coil 52 is mounted on the long leg of a wound core53 which is cut at its corner at 54 according to the above method, butinstead of being forced together by a non-magnetic strap clamp l3, asuitable non-magnetic spacer 55 is placed in the saw cut to interruptthe magnetic circuit by an air gap. This is a choke coil reactor ofcommon type.

Figure 11 illustrates a further transformer core of the shell typewherein the common coil 56 is mounted to surround the adjacentcontiguous legs 51 and 58 of separate wound cores l I. In thistransformer the adiacent inner corners have diagonal butt joints at 59and GI. During assembly the legs 51 and 58 are held tightly togetherwhile the remaining parts of the two core ele ments are spread apart topermit the insertion of coil 56 over the two legs. Then the wholeassembly is clamped together by the steel strap 62 and fastening means63. Suitable upper and lower metal blocks 64 contoured to strap 62 andthe arcuate corners of the core elements are inserted between strap 62and the core. The block 54 at the top of the transformer core bridgesthe open ends of both the butt Joints 59 and 6| and in cooperation withthe clamp strap 62 helps to maintain the butt joints more compactly andtightly in the assembly.

The method of winding the core illustrated in Figure 12 is essentiallythe same as in Figure 3 except that the winding head I9 is disposed in asuitable furnace 65 having an entrance throat 66 wide enough toaccommodate strip 22 during winding. The purpose of this furnace is toheat strip 22 prior to and while winding on block l8 so that the coreturns may be given a permanent shape concommitantly with the windingoperation, whereby the separate annealing operation may be eliminated.

A long gas burner or equivalent heater 61 is mounted to heat the strip22 as it enters the fur-- nace so that it is wound hot about block l8and therefore readily bends and conforms itself to the arcuate cornersof the wound core. If desired burner 61 may be replaced by an electricalinduction heater. Preferably I maintain a temperature of about 1700" F.in the zone of heater 61, but this must be correlated with strip windingspeed to obtain the optimum condition in each furnace. Preferably a pairof further burners or like heaters 68 are provided for applying localheat to the core during winding, the most heat being applied to thelower corners when the core is in and near the position of Figure 12.These heaters 68 prevent the core from chilling too swiftly and insurethat each turn at the corner is heated to 1700 F. while conforming tothe contour of the core corner thus effectively reproducing conditionsin the separate annealing furnace.

ment of the invention it is particularly essential that the windingtension be progressively decreased as the core turns accumulate because,as the succeeding hot wound interior turns cool. they contract and buildup increasing tension in the succeeding turns which must be compensatedby a corresponding correlated decrease in winding tension. Thu th striptemperature, winding speed and tension must all be correlated in thisembodiment to obtain optimum results.

My invention provides a wound core of magnetic strip material that iscut at only one place during assembly with a coacting coil so that itcontains only one smoothly and tightly closing butt Joint with pressureon the lamination ends at the joint being substantially equallydistributed for producing a minimum of vibration of the laminations andminimum exciting current losses. The efficiency of my core in the finalassembly compares favorably with uncut cores, the losses and excitingcurrent in my core being no more than 10 percent greater than in uncutcores.

My improved method of winding the core wherein the winding tension isgradually reduced as the core turns build up enables the manufacture ofa practical wound core that is less expensive and represents a saving inmaterial and labor over punching and other core forming methods. In mymethod the reduction of winding tension with the accompanying reductionof inner surface pressures between the accumulating turns insuresagainst contraction or relative slipping of the outer layers,particularly at the rounded corners, after the core is out.

If desired two or more strips of the same or different width or magneticqualities may be simultaneously or successively wound on form i8. Forexample successively wound strips of different width may be employed toproduce a cruxiform core usable in some transformers. Each strip sowound is controlled as above explained.

The preferred embodiment of the invention illustrates a straight sidedoblong core to which the invention is peculiarly adapted, especiallywhere the coil window is long. The invention is particularlyadvantageous in core and coil assemblies of high electrical efficiencywhich demand a coil window length at least four times the core thicknessand a coil window width of the order of twice the core thickness,usually not more unless the window is proportionally longer. As apractical consideration of both good transformer design as well as themechanical characteristics of this core, the window should be at leastthree times as long as it is wide. When the core is closed and clampedtight all the ends of the respective turns are held tightly closed atthe joint except the first turn on the inside and the second turn whichsometimes fail to lock securely against the adjacent end in which casethey must be supported and restrained from vi- I have discovered thatthe winding tension of strip 22, in the method illustrated in Figure l2.must be maintained relatively low during the entire operation so as notto stretch the hot steel strip or reduce its cross section.

In m d embOd brating by pressure of the coil assembly and the insulatingstrip [8 of Figure 1. In case the coil and insulation strip do not clampthis inner layer, its vibration may be urther prevented by providing asuitable thin metal clip 40 extending laterally across the inner turn ofthe core adjacent the inner end of joint i5 as illustrated in Figure 6.Clip 40 has its opposite ends inturned and interleaved with the innerseveral turns so that the latter are bound together and given such massthat they do not vibrate in normal operation. No clip is needed in theshort 13 side laminations because of their low leverage with respect tothe adjacent corner.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. The method of forming a hollow wound core of magnetic strip materialcomprising the steps 01, winding a length of said strip material on aform having two substantially parallel straight sides, with the tensionin said strip prior to its disposition on the already accumulated turnson said form decreasing gradually as the turns accumulate, clamping thewound length of strip tightly along said two sides, annealing saidclamped wound strip, clamping one of said straight sides and thecontiguous side at their juncture to tightly compact said wound stripsthereat, and cutting said wound strips at said juncture between saidclamped portions.

2. The method of forming a magnetic core 30 and coil comprising thesteps of, winding a length oi strip material under gradually decreasingtension on a form having two substantially parallel straight sides,clamping the wound length of strip tightly along said two sides,annealing said clamped wound strip, clamping one of said straight sidesand the contiguous side at their juncture to tightly compact said woundstrip thereat, cutting said wound strip at said juncture between theclamped portions, springing said straight side away from said contiguousside, placing a coil over said straight side, and applying ressurecompletely around the periphery of said wound strip and assembled coilto close said out substantially across its full area.

ULYSSES S. DUNN.

References Cited in the tile of this patent UNITED STATES PATENTS NumberName Date 1,935,426 Acly Nov. 14, 1933 2,264,800 Horstman Dec. 2, 19412,313,306 Wiegand Mar. 9, 1943 2,387,099 Vienneau Oct. 16, 19452,393,439 White et a1 Jan. 22, 1946 2,408,211 Hodnette Sept. 24, 19462,416,989 Gauthier Mar. 4, 1947 2,478,029 Vienneau Aug. 2, i949 FOREIGNPATENTS Number Country Date 7,856 Great Britain Mar. 15, 1890

